Preparing images for large format digital printing

Learning about colour management and how it can help prepare images for large-format printing processes is the only way to avoid these pitfalls.

As I write, I am spending a few hours in an airport, an appropriate place to evaluate the state of today’s signage, with its combination of backlit, vinyl, and paper-based displays. Most of the images are printed very well, but a few have problems with contrast, poor colour reproduction and image resolution.

The biggest surprise is a large backlit ad for a German automotive company, presenting their flagship car. The chassis of the vehicle is a rich black, but the gravel road, normally grey, has taken on a strong green shade.

Grey is one of the strongest ”memory colours” (colours with a preconceived context) for viewers, and must remain neutral. It has very little latitude to be presented in the wrong fashion, compared to other “memory colours”, such as “fire truck” red or “sky” blue. For most people, seeing the car driving along a green gravel road will take away from the intended emotional response: a desire to own and drive the car.

Calibration and profiling

The large-format print must depict what the client wants. Therefore, signmakers must be able to trust what they see on their computer screen to be accurately reproduced when printed, or they run the risk of wasting time, material and ink and losing customers.

The way to achieve this holy grail of print production is the proper calibration and profiling of your monitor, which means setting it to specific industry standards for brightness, contrast, gamma and colour. Accepted values for printing and sign making are: .4 for black, 95 for the brightness of white, a 2.2 gamma and 6500K for the colour temperature of white. Re-calibrating is simply resetting the device to these established values every 30 days or so.

Profiling your monitor measures how your particular monitor interprets colour. Every monitor is different, and applications like Photoshop need a description of how a monitor sees colour so it can transparently compensate for your monitor’s limitations.

Today’s monitor calibration packages, costing from $250-$500, are much more user-friendly than previous editions, and work with all platforms and monitors.

Soft-proofing

With Photoshop’s soft-proofing feature, the on-screen appearance of an image is adjusted to match how it will look when printed, without changing the pixel values within the image.

Frequently an image is viewed on a calibrated and profiled monitor that provides more contrast and a wider colour gamut than the corresponding print media can reproduce. Soft-proofing capabilities allow the image to be ‘dumbed-down’ on-screen to match the narrower gamut of the output device.

Another feature called “paper white” allows the user to match the media’s white point. The ‘catch’ with this option is the need for an accurate International Color Consortium (ICC) profile describing the device to be targeted (see Figures 1 and 2).

Image optimization

There are numerous options available for adding contrast to an image, with the most obvious being the brightness and contrast controls. However, the danger of this feature is it discards highlight and shadow detail. A better option is to use the levels or curves option, which allows images’ tonal range to be maximized without losing any important detail.

This process can be carried out by opening levels and looking at the image’s histogram. If the image is lacking in contrast, the histogram will resemble the image in Figure 3, with gaps at one or both ends of the histogram.

Moving the triangular icons in to meet the black edges will reset the white and black points. If the image’s black point is at 85 per cent, for instance, the image will look flat. Instead, the blackest point in the image should be 100 per cent black.

It is important not to go past the edge of the histogram. Doing so will only darken areas of the image where important detail can be lost.

An alternative method to add contrast involves the “unsharp mask”(USM) filter. Using this with the values shown in Figure 4 will add an additional level of contrast to an image. Increasing the values amplifies the effect, but it is important not to overdo it, to avoid ‘halos’ around images.

Traditional image sharpening with the USM tool is still required and should be performed as a final step. Traditional values for the USM filter are:

Amount: 75 to 150

Radius: 0.7 to 1.5

Threshold: 0 to 5, for a 300 pixel-per-inch (ppi) image

Optimal base resolution for large format printing

Viewing distance plays a major role in determining the required resolution. An alternate method to determine the best image resolution is to first determine how far away the average viewer will be viewing your print. You can reduce the resolution by a third for every 6 feet we move away from a print.

One method to determine the optimal image or base resolution required for an image file in large-format printing is to divide the output resolution by three. For example, if an output device can print at 360 dots per inch (dpi), 120 dpi is the required image or base resolution for your file printing on a device that prints at 360 dpi.

For example, starting with 360 dpi as maximum resolution, an image to be viewed from 1.8 m (6 ft) away could be printed at 240 dpi. If the viewing distance were 3.6 m (12 ft), however, the image’s resolution could be lowered to 120 dpi, because the human eye cannot see 360 ppi of detail from 3.6 m (12 ft) away.

File preparation tips

1. Convert all fonts to curves. Include the fonts with the file, in case a small edit is required later.

2. Using Photoshop’s convert to profile feature, convert bitmapped images (photos) to the U.S. Sheetfed Coated V2.icc profile. Embed the profile when saving.

3. Use the Tag Image File Format (TIFF) with bitmapped images whenever possible.

4. All images must be in the cyan, magenta, yellow and black (CMYK) colour mode. Lamda and LightJet devices are exceptions that require a red, green and blue (RGB) colour space.

5. Use vector files whenever possible.

6. Always supply a hard-copy colour proof.

7. All paths should be drawn by hand. Automated path creation options are overly complex and will often cause errors in processing.

8. Specify all solid colours using the Pantone Coated ‘C’ library.

9. Avoid using transparency in Adobe Illustrator.

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